Skid control systems currently in use today typically comprise one or more wheel speed sensors coupled to the wheels of a vehicle to monitor the rotational speed of the wheels and produce output signals, the frequencies of which are proportional to the speed of the wheels. These signals are provided to a tach circuit which converts the sensor signal to a d. c. analog signal proportional to the frequency of the signal received at its input. The output from the tach circuit is then supplied to the performance circuit of the skid control system where it is analyzed to determine the existence of an incipient skid condition.
In systems of this type, it has been discovered that, under certain circumstances, it is possible for the tach circuit to erroneously indicate to the performance circuit that a skid condition is present. Specifically, when a vehicle is moving at a slow rate of speed or is stopped on an inclined surface, it is not uncommon for the brakes to emit a relatively loud "squealing" noise as they are gradually applied or released. This is especially true in the case of a large vehicle, such as a truck. When this occurs, the noise causes the wheel sensors to produce an output signal in response thereto that appears to be the tach circuit as a rapid wheel speed acceleration. When the noise terminates, the resulting sudden drop in the sensor output signal is interpreted by the performance circuit as a rapid wheel deceleration, indicating imminent wheel lock-up. The circuit responds, therefore, by releasing the brake pressure, which results in inadvertent loss of the brakes.
Accordingly, it is the primary object of the present invention to prevent the inadvertent release of brake pressure caused by the skid control system responding to brake noise. In general, the present invention incorporates into the skid control system a switched filter circuit which is activated when the vehicle is traveling at speeds below a predetermined rate -- e.g. 5 miles per hour -- and is adapted to exclude from the tach circuit all signals above a predetermined frequency -- e.g. 470 Hz. The circuit operates on the premise that at low speeds no high frequency signals should be present. Therefore, if such a signal is, in fact, present, it is presumed to be noise and is ignored.
The preferred embodiment additionally includes an override circuit for preventing the switched filter circuit from becoming activated during certain periods of operation of the skid control system while in an actual skid condition. This avoids the possibility of the switched filter causing excessive brake release under certain extreme conditions which will be fully explained in the following detailed description of the preferred embodiment.